The invention relates to a charge-coupled device for converting electromagnetic radiation into discrete electrical charge packets and transporting these charge packets for reading them out. This device includes a semiconductor body having at least three successive layers from a surface in a direction transverse to the surface: a first layer of a first conductivity type, which constitutes the charge transport channel of the charge-coupled device; an adjoining second layer of the second conductivity type for forming a potential barrier, over which excess charge carriers generated in the case of local overexposure can flow in a direction transverse to the surface; and an adjoining third layer of the first conductivity type for draining excess charge carriers. The surface is provided with a system of electrodes which are connected to means for supplying clock voltages varying between active and blocking levels, with potential wells and potential barriers, respectively, being induced in the subjacent transport channel. The invention further relates to a camera provided with such a charge-coupled device.
Such a device is generally known inter alia from the article "A Frame-Transfer CCD Colour Imager with Vertical Anti-Blooming" by M. J. H. v.d. Steeg, et al, published in I.E.E.E. Transactions on Electron Devices, Vol. ED-32, No. 8, Aug. 1985, p. 1430-1438.
A charge-coupled device can form together with a large number of similar devices a bidimensional image sensor for use in, for example, a camera. In such sensors, it is common practice to take measures which prevent, in the case of local overexposure, generated charge carriers from spreading over the sensor beyond the overexposed area (pixel) itself. These measures are often designated in literature by the term "anti-blooming". In the best known method of "anti-blooming", so-called overflow barriers and drain zones, by means of which the excess charge carriers can be drained, are provided between the columns of the bidimensional pixel pattern at the surface of the semi-conductor body. Since, due to this anti-blooming method, the resolution and the sensitivity are reduced, the less conventional vertical anti-blooming (VAB) has also been suggested, inter alia in the afore-mentioned publication. This publication describes a sensor having a vertical npn configuration, in which the upper n-type layer constitutes the buried channel of the charge-coupled device, the p-type layer constitutes the overflow barrier for excess charge carriers and the lower n-type layer constitutes the drain of the excess charge carriers. The intermediate layer, here the p-type layer, is preferably provided below the electrode below which the charge is collected with a part of reduced thickness or even with an opening, through which the lower n-type layer adjoins the upper n-type layer. In such a configuration, "blooming" due to overexposure can effectively be avoided without or at least substantially without any detrimental effect on the resolution and/or the sensitivity.
If in this device the extent of overexposure to be handled should be increased, i.e. the extent of overexposure with which all the excess charge carriers can still be drained, it is most obvious to reduce the overflow barrier between the charge transport channel and the drain layer. This may be effected by increasing the voltage at the lower n-type layer (the substrate) and/or by increasing the voltage externally applied to the p-type layer. The increase of the substrate voltage has the disadvantage that a higher maximum supply voltage may be required. However, a more essential disadvantage is that the reduction of the overflow potential barrier leads to a decrease of the maximum charge packet, as will be explained more fully in the accompanying description of the Figures. The decrease of the maximum charge packet to be handled per pixel in general leads to a reduction of the signal-to-noise ratio, which in turn results in a deterioration of the picture quality during display.
Another method of increasing the maximum overexposure to be handled could consist in that the blocking voltage--in an n-channel device--is reduced and hence the potential barrier below the relevant electrodes is increased. If the overflow potential barrier to the substrate is then kept constant, the overexposure can be increased and hence the potential level of the charge packet can also be increased without charge carriers flowing to adjacent pixels, due to the higher potential barrier below the blocking electrodes. In practice, this enlargement of the voltage sweep of the clock voltage is not possible or desirable because the dissipation would become too high.